Handling vmentry failures in userspace

Handling vmentry failures in userspace

[Mohammed Gamal]

With the unpredictable behaviour of the current vmentry failure patch,
I think this is caused by external interrupts from the host, and since
this is hard to control I was thinking of handling vmentry failure in
userspace. The way I am thinking of doing it is that once a vmentry
fails because of an invalid guest state we delegate the emulation to
userspace, and then we'd rely on QEMU's CPU emulation to emulate
instructions until we return to VMX-friendly state.

I don't have a strong grasp of the details on how the kernel- and
user-space side interact, so I don't know if that'd really be
possible? If it is, is it plausible to do it this way, or is it better
to leave all handling within the kernel side?

Re: Handling vmentry failures in userspace

[Avi Kivity]

Mohammed Gamal wrote:
> With the unpredictable behaviour of the current vmentry failure patch,
> I think this is caused by external interrupts from the host, and since
> this is hard to control I was thinking of handling vmentry failure in
> userspace. The way I am thinking of doing it is that once a vmentry
> fails because of an invalid guest state we delegate the emulation to
> userspace, and then we'd rely on QEMU's CPU emulation to emulate
> instructions until we return to VMX-friendly state.
>
> I don't have a strong grasp of the details on how the kernel- and
> user-space side interact, so I don't know if that'd really be
> possible? If it is, is it plausible to do it this way, or is it better
> to leave all handling within the kernel side?
>   

Userspace emulation doesn't work.  Some of the emulated devices are in 
the kernel; getting smp right in userspace is hard; and most importantly 
doing some of the cpu emulation in userspace means that the boundary 
between the kernel and userspace is blurred, making it difficult to 
write alternative userspaces.

Fixing the problems we have may be hard, but we need to do this right.

-- 
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